In general, in order to determine a flow rate from a measured transit time of a thermal pulse, a thermal pulse emitter such as a heater resistance element is placed in the flowing fluid. A temperature sensor, e.g. a thermocouple, is placed in the proximity of the thermal wave generating element.
The time taken by the thermal pulse to propagate from the emitter to the sensor is measured. This transit time depends on the flow speed of the fluid. When the shape of the flow duct is known, the flow rate can be deduced from the measured flow speed.
It is conventional practice for the thermal pulse to be modulated. Under such circumstances, it is a phase shift between the emitted thermal wave and the detected thermal wave that is, in fact, measured.
In this specification, measuring "transit time" is used generically to cover both measuring a phase shift between the emission and the detection of a thermal pulse and measuring a propagation time of that pulse.
That basic principle applies only in the ideal case where the temperature, the pressure, and the composition of the fluid are not subject to change. Should any of these parameters fluctuate, then the diffusion coefficient of the fluid varies, and for given flow speed, pulse propagation time varies. Flow rate measurement thus contains error. Unfortunately, these parameters vary considerably in the fuel gas normally delivered by suppliers. Thus, under such conditions, in order to be able to determine flow rate by using transit time measurement (propagation time or phase shift), it is necessary to correct the measured transit time to take account of variations in temperature, pressure, and composition of the gas. Knowledge of these parameters makes it possible to determine the diffusion coefficient of the gas, and consequently to deduce the speed of the fluid (and its flow rate) on the basis of a measured transit time.
It will be understood that such additional measurements make the determination of flow rate much more complicated and require devices that are difficult to implement and that are expensive both in money and in energy.
U.S. Pat. No. 4,713,970 discloses a device for measuring propagation time that is compensated in temperature and in pressure while avoiding taking direct measurement of temperature and pressure. A wire heating resistance is deposited on an insulating substrate and a thermo-electric detector is placed on the substrate to detect the thermal waves emitted by the heating resistance. The hot wire extends perpendicularly to the flow direction of the fluid, and the thermoelectric detector is placed in the fluid flow directly upstream or downstream from the hot wire so as to detect thermal waves that propagate parallel to the fluid flow. A reference device identical to the above device is used for compensating for the effects of pressure and temperature. For this purpose, the reference device is used to perform measurement at a zero flow rate.
In a first series of embodiments, the reference device is placed perpendicularly to the measurement device. Thermal wave propagation is then substantially perpendicular to the fluid flow direction and is thus practically independent of fluid flow speed. However that device is sensitive to disturbances in the flow.
In another embodiment, the reference device is placed in a region where the fluid is at rest.
Although the devices described in Document U.S. Pat. No. 4,713,970 do indeed provide improvements over the basic device, they nevertheless suffer from numerous drawbacks that prevent them from being used in an application to metering fuel gas.
The heater element is placed on the same wall of a substrate as the thermoelectric detector. Thermal pulse propagation measurements are performed locally in the vicinity of the wall and are therefore highly sensitive to variations of viscosity due to possible changes in the composition of the fluid. This measurement inaccuracy is inherent to the structure of the sensor and cannot be compensated without additionally determining the composition of the fluid. In addition, in the vicinity of the substrate wall, the dynamic speed of the fluid is reduced relative to the dynamic speed at some distance from said wall. It is known that flow speed is substantially zero along a wall whatever the speed profile may be elsewhere. This therefore results in the sensitivity of the system being reduced. Furthermore, since the substrate is not a perfect insulator, thermal leaks exist between the thermal emitter and the thermal sensor. Such thermal leaks necessarily increase the amount of energy that needs to be provided to ensure that the system operates properly. Variations in pressure and temperature are compensated in U.S. Pat. No. 4,713,970 by doubling up the measurement structure. As a result, the devices described therein require at least two heater elements.
It will thus be understood that the energy required to operate such compensated devices is at lease twice the energy that is required for a device having a single heater element. Unlike the field of measuring the air flow in automotive engines induction, which is the field addressed by the devices of the document U.S. Pat. No. 4,713,970, questions of energy consumption are of great importance in the field of obtaining a volume flow meter suitable for metering fuel gas. It is then necessary for an electrical battery to be capable of providing sufficient energy to guarantee a lifetime of at least ten years without there being any need to change the battery. This constraint makes it impossible to adapt one of the devices described in document U.S. Pat. No. 4,713,970 to a gas metering application.